Ohmic standard apparatus

ABSTRACT

An ohmic standard apparatus having five rocker arm type switches and four resistors associated with each switch. The switches provide a digital readout of the total resistance for resistance settings from 0 to 99,999 ohms. A slide wire type resistor is interconnected with the resistors to provide resistance settings up to 100,000 ohms.

0 United States Patent [1113,571,778

[72] 1nventor John P. Smith, Jr. 2,498,967 2/1950 Sehaefer 338/200XElton, Pa. 2,625,632 1/1953 Onia et a1. 338/129X [21] App1.No. 741,4102,678,985 5/1954 Smith,Jr. 200/11(D)X [22] Filed July 1,1968 2,786,1223/1957 Strain 338/134X [45] Patented Mar. 23, 1971 2,886,677 5/1959Bourns 338/130X [73] Assignee Vishay Inter-technology, Inc. 3,017,5651/1962 Carson et a1. 338/128X Malvern, Pa. 3,215,790 11/1962 Young 200/11(D) 3,474,375 10/1969 Smith, Jr. 338/231 54 OHMIC STANDARD APPARATUSFOREIGN T 6 Claims 5 Drawing Figs. 517,541 1940 Great Britain 338/231 52us. or ass/200 [51] Int Cl I no 1c 16 Attorneys'1homas M. Ferrill, Jr.and Roger Norman Coe 50 Field ofSearch 338/134,

[56] References Cited UNITED STATES PATENTS 330,244 11/1885 Lange338/129 338/200 2,286,029 6/1942 Van Beunen ABSTRACT: An ohmic standardapparatus having five rocker arm type switches and four resistorsassociated with each PATEN-TED m2 3 1911 saw 1 or 2 INVENTO Jain K anus2,1.

ATTORNEYS OHMTC STAND APPARATUS C ROSS-NCE TO RELATED APPLICATIONSBACKGROUND OF THE INVENTION The present invention relates to ohmicstandard apparatus and more particularly, to ohmic standard apparatus ofextremely high accuracy, stability and versatility.

Conventionally, separate standard resistors, each having a single value,are employed for precise resistance measurements. A single laboratorystandard resistor may be l /zto 3 inches in diameter and may be severalinches in height. A laboratory set of such resistors could consist ofseveral carefully chosen resistors having values in the fraction ofan-ohm range, several carefully chosen resistors having values in theunit ohm range, several carefully chosen resistors having values in theunit ohm range, several carefully chosen resistors having values in therange of tens of ohms, etc. Thus, in the past it has been necessary tohave several shelves full of individual resistors in order to makeprecise resistance determinations. The use of these standard resistors,each having a single value, necessitates difficult and time consuminginterconnections. I

For applications in which the accuracy of standard resistors are notrequired resistance decade boxes have been used. Virtually all availableconventional and specialized resistance decade boxes are subject to suchart recognized problems as:

a. lack of instrument accuracy;

b. resistance shifts caused by load;

0. changes with temperature, moisture, or the pressure of surroundingmedia (e.g., atmospheric pressure); and

d. difficulty in the measurement of final digits.

The last problem is of particular importance sinceit constitutes aninherent limitation with resistance decade box performance as to usablefrequency, size and ease of setting. The conventional approach to theperformance of resistance decade boxes centers on the accuracy ofdiscrete resistors (usually, inductive resistors) used in the boxes.This approach creates considerable confusion since the user has to addor subtract compensation values which often differ with various rangesof resistance values.

SUMMARY OF THE INVENTION formance parameters, including accuracy,stability and versatility.

Another object of the present invention is to provide apparatuscombining in a compact unitary structure, extreme reliability forprecise resistance determinations to many significant figures with theflexibility, speed of use and convenience for changes of resistancevalues comparable to those afforded (without extreme accuracy) byresistance decade boxes.

Still another object of the present invention is to provide ohmicstandard apparatus which has virtually infinite resolution in resistancesettings up to 100,000 ohms.

Yet another object of the present invention is to provide ohmic standardapparatus in which changes of total resistance can be made simply and inwhich these changes can be read directly from the apparatus.

In accordance with the present invention, ohmic standard apparatushaving noninductive precision resistors are connected to compactrocker-arm type switches which are arranged for digital readout of totalresistance for resistance settings from to 99,999 ohms. As an integralpart of such apparatus a slide wire type resistor is interconnected withthe aforementioned noninductive precision resistors to provide infiniteresistance settings up to 100,000 ohms. The slide wire type resistor isinterconnected to a vernier dial which is marked in 0.01 ohm divisions.

Thus, the ohmic standard apparatus of the present invention overcomesthe bulk and interconnection problems of standard resistors. Moreover,the ohmic standard apparatus of this invention has the convenience ofoperation of conventional resistance decade equipment while retainingthe accuracy of standard resistors.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, as well as its objects,advantages, features and aspects, will be more readily understood fromthe following detailed description thereof, when considered inconjunction with the drawings, in which:

FIG. 1 is a plan view which illustrates the faceplate of one embodimentof the present invention;

FIG. 2 illustrates the interconnection of the circuitry, resistors andswitches which is mounted on the back of the faceplate shown in FIG. 1;

FIGS. 3 and 4 are fragmentary views illustrating opposite sides of therocker actuated resistance switches shown in FIGS. l and 2; and

FIG. 5 is a graph showing the deviation from various resistance valuesat various frequencies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand particularly to FIG. 1, ohmic standard apparatus 10 is shown housedin a sturdy insu- Iated case 11. The faceplate 12 of the apparatuscontains five ten-position rocker actuated switches, respectively 13,14, I5,

16 and 17, having direct numeric readout 19 of total resistance valuesranging from 1 to 99,999 ohms. In addition, the embodiment illustratedby FIG. 1 has a vernier dial 20 mounted on faceplate l2 and marked with0.01 ohm divisions. Thus, the combination of switches 13 through 17 anddial 20 on apparatus 10 permit resistance values to be set (as describedbelow) and read in 0.01 ohm divisions up to 100,000 ohms. Faceplate 12is also provided with terminals 22 and 23. A third terminal 24 can beprovided, as required, for grounding.

Referring to FIG. 2, the interconnection of the circuitry, resistors andswitches mounted to the back of faceplate 12 is shown. The circuitrycomprises interconnection 26 between terminal 22 and switch 13;interconnections 27, 28, 29 and 30 between switches 13, 14, 15, 16 and17; interconnection 32 between switch 17 and a slide wire type resistor34; and interconnection 33 between resistor 34 and terminal 23. Resistor34 comprises two wires 36 and 37 interconnected at one end and mountedon drum 38. Movement of dial 20 turns drum 38 thereby increasing ordecreasing the length of wires 36 and 37 in relation to pressurecontacts 40 and 41. Thus, resistor 34 is a slide wire type rheostat inwhich its resistance value is determined by the total length of wires 36and 37 between contacts 40 and 41. Contacts 40 and 41 are connectedthrough conductive strips 43 and 44 with interconnections 32 and 33.

The rocker actuated switches l3, 14, 15, 16 and 17 have bidirectionaloperation. Switch 13, illustrated in FIG. 3, is a typical example ofthese switches. Relatively large switch pads 46 mounted on a rotatinginsulator 47, are provided on one side of switch 13 to assure uniformlylow switch resistance. Preferably, pads 46-46 are gold plated tomaintain contact resistance at a minimum. Actuation of switch 13 rotatespads 4M6, thereby bringing varying contact arms 48-48, attached tostatiohary insulator 50, into contact with pads 456-46. Leakage isvirtually eliminated through the use of insulators 47 and 50.

As illustrated in FIG. 4, the contact arms 4M8 shown in PEG. 3 areconnected with noninductive precision resistors 52-52. Examples ofnoninductive precision resistors which can be used are those describedin the aforementioned Zandman et al. and Smith applications, which arehereby incorporated by reference. In accordance with the Zandman et al.

application, a high-precision resistor is constructed by supporting athin film of a selected metal alloy upon a substrate having knownphysical properties, the substrate being many times thicker than themetallic film (preferably, of the order of 100 to 1,000 times thicker).The metallic film is caused to have a predetermined pattern, such thatelectric current flows along a conducting path of very great length andextremely small width, this pattern preferably including a great numberof parallel narrow linear path portions in a planar array. The side ofthe substrate having thereon the resistor film of predetermined patternis coated with an epoxy resin, and the opposite side of the substrate issimilarly coated to equal thickness with epoxy resin of the same kind.These oppositely disposed coatings of epoxy resin provide symmetry withrespect to their mechanical effects upon the substrate, so as to avoidany tendency to cause bending thereof. Similar balance can be obtainedwith unequal coatings of different characteristics. Moreover, the epoxyresin on the conductive layer reinforces the resistive path and protectsit from mechanical damage, and also protects it from any skin acidswhich might otherwise reach it in the course of being handled.

In one embodiment, a resistor of the Zandman et al. applicationcomprises a substrate having an etched-pattern resistor layer of bulkmetal film fixed to one surface thereof. Coatings of a hard epoxy resinare applied to the resistor surface and the opposite surface of thesubstrate. The two opposite epoxy coatings are so related to each otheras to result in a sandwich which does not bend or warp as a consequenceof changes of temperature or moisture absorption by the coatings.

The substrate may be made of glass having a temperature coefficient ofexpansion of the order of 3 parts per inillion per degree F. In anillustrative resistor unit, the substrate may be of the order ofone-fourth inch by one-fourth inch with a thickness of 0.04 inch.

The bulk metal film may be made from a resistive alloy such as one ofthe Nichrome alloys, wherein nickel and chromium are the principalmetals. This film may be of the order of 0.0001 inch thick.

The metallic film is photoetched to a pattern which establishes a narrowconductive path of much greater total length that the dimensions of theface of the substrate. This step may be carried out after the film hasbeen bonded to the substrate as by the layer of plastic thereunder, orit may be carried out when the metallic film is on a thin support suchas a plastic layer but before being bonded to the substrate. To performthe photoetching, the face of the thin alloy film opposite the plasticlayer is covered with a photosensitive masking medium such as KodakPhoto-Resist (KPR). By means of a photographic exposure and development,the KPR is retained in contact with the surface of the film only in thedesired resistor pattern and is removed from those portions where thealloy film is to be etched away. An etching process is then used toremove the exposed portions of the thin alloy film.

The pattern in which the film is exposed and etched may include severalwider portions and portions of shorter lengths, so that an operator isenabled to complete the steps of bringing about the desired resistancevalue with the inclusion of one or more of the lower-resistanceincrements as may be needed.

With the alloy film etched in the desired pattern and bonded to onesurface of the substrate as by a suitable epoxy bonding cement, furtherepoxy material is added, covering the surface of the metal film. Thisprecedes the adjusting of the resistance by cutting, referred to above.As the cut is being made through the outermost plastic and at least partway through the alloy film, either with a stylus or with an abrasivecutter or other tool, the plastic holds the film in position along theboundaries of the cut and resists any tendency for the film to bedetached from the substrate.

The upper protective epoxy coating may have a total thickness of 0.00linch. There is also applied to the opposite face of the substrate afurther epoxy coating. The epoxy coatings may be of equal thickness andidentical characteristics in order that the stress contributions whichthey make to the flat surfaces of the glass substrate shall be balanced,and shall not tend to cause bending or warping of the glass substrate.Along with this elimination of bending, any tendency toward longtermdimensional instability due to stress relaxation is substantiallyovercome. Alternatively, the same result can be obtained by coatings ofdifferent material charac-.

teristics provided that their thicknesses are properly related.

The glass substrate has a temperature coefficient of expansion of theorder of 3 parts per million per degree F. The epoxy or other plasticcoatings on top and bottom of the glass substrate have a much highertemperature coefficient of expansion, of the order of 40 parts permillion per degree F. Furthermore, said epoxy coatings tend to expand orcontract as their moisture content varies. Hence, the balancedapplication of the epoxy or other plastic to both sides prevents it fromcausing bending of the device.

The modulus of elasticity of the glass substrate is many times higherthan that 10 the epoxy material. Hence, the expansion and contraction ofthe unit in length and width are determined mainly by the temperaturecoefficient of expansion of the glass. Inasmuch as the total thicknessof the epoxy layers in the described embodiment is of the order ofonetwentie'th the thickness of the glass substrate, and the modulus ofelasticity of the epoxy is of the order of one-thirtieth of the modulusof elasticity of the glass substrate, the tendency of the epoxy toexpand with temperature by a factor of IO times greater than theexpansion of the glass is made comparatively small by the relativethinness of the epoxy material and its far lower modulus of elasticity.

The resistive alloy film, etched in its predetermined pattern and bondedto the glass substrate, being of the order of one hundredth to onethousandth the thickness of the glass, exerts minimal influence upon thedimensional responsiveness of the unit to the changes of temperature andmoisture.

By the selection ofa nickel chromium alloy with such minor alloycomponents as to provide a desired curve of resistivity versustemperature and a desired temperature coefficient of expansion, theresistor may be made to have a reliable temperature coefficient ofresistivity as low as l part per million per degree C. in the vicinityof a desired design temperature such as 25 C., and to have an extremelylow overall temperature coefficient of resistivity throughout a rangefrom -55 C. to +1 75 C. In general, the alloy consisting primarily ofnickel and chromium will have a greater temperature coefficient ofexpansion than the glass substrate. Hence, with increasing temperature,as the glass substrate elongates and carries with it the alloy filmlayer, the alloy film is subjected to compressive stress. Conversely, asthe glass substrate contracts with decreasing temperature and the alloylayer tends to undergo greater contraction, the resistive metallic filmwhich is bonded to the glass and constrained to duplicate thecontraction of the glass is subjected to tensile stress.

Provided that the net sum effect of the resistance change component dueto changing stress in the alloy film and the resistance change componentdue to expansion or contraction of the film is substantially equal tothe temperature coefficient of resistivity of the alloy understress-free conditions, and of opposite sign, the overall temperaturecoefficient of resistivity of the device is substantially zero. Sincethe last-named factor is not linear, the device will have a predictablevariation of its temperature coefficient of resistivity throughout thedesign temperature range.

The resistor may be encapsulated in a plastic or metal housing whereinsuitable potting material or materials are included to embed one or moreresistor units such as that described above. In order to protect theresistor unit against any substantial mechanical forces exerted by orthrough the potting material, the resistor unit is provided with asheath of soft rubber, polyurethane foam, or other very soft material.Such soft material may be used alone, filling the space surrounding thecoated substrate, if desired; alternatively, the soft material may inturn be surrounded by a hard filler such as an epoxy. The soft material,with a thickness preferably many times greater than the thickness of theepoxy layers on the substrate, serves as a protective cushion by virtueof its very low modulus of elasticity.

By virtue of the soft cushion and flexible conductors, the dimensionalchanges in the hardened potting material, which may be of the order of 5to times greater than the dimensional changes which the resistor unititself tends to undergo, are isolated and prevented from forcing theresistor unit to depart from its design characteristics.

in accordance with the teaching of the aforementioned Smith application,the resistor element consisting of the substrate and the resistivemetallic film thereon, which has elastic properties obeying i-iooks lawin tension and compression, is immersed within a selected oil and housedin a hermetically sealed casing, the arrangement of the resistor elementbeing such as to minimize the transmission of any mechanical forces tothe element from the housing.

Specifically, a metallic cylinder is arranged to receive two end pieces,each of which consists of a glass disc having a surrounding metal ringfixed to its periphery by a glass-to-metal seal. Each disc also includesa metal eyelet centrally located therein, and similarly bonded to theglass by a glass-to-metal seal. The surrounding metal ring of each endof the disc is arranged to be bonded by solder at its periphery to anend of the metal cylinder. Preferably, a minute shoulder is formed ineach end of the metal cylinder to facilitate the accurate positioning ofthe end disc with its surrounding metal ring. The central eyelet of eachend disc is also arranged to project a short distance from the surfaceof the glass and is prepared to be bonded by solder to a terminal wirelead of the unit.

As one example of mutually compatible glass andmetal materials,borosilicate glass can be used with kovar metal.

The assembly to be housed within the cylinder between the end discsconsists of a very small printed circuit board upon which is supported,by its flexible leads, a resistor element comprising a rigid dielectricsubstrate having a resistive metallic film, having elastic propertieswhich obey l-looks law in tension and compression, afiixed thereon. Thearrangement and construction of the resistive element, arranged for arelatively long conductive path between the junctions of the flexibleleads is described in the aforementioned Zandman et a1. application.

Also fixed to the printed circuit board are wires which are arranged toserve the dual purposes of supporting the printed circuit board, andconstituting the ultimate terminal wires of lead wires of the completedresistor.

The enclosure, consisting of the metal cylinder and the end discsperipherally bonded to the cylinder and centrally bonded to the leadwires is almost entirely filled with a suitable oil, a suitable materialfor this purpose having been found to be Dow Corning 200 Silicone Oil. Avery small pocket of gas, such as dry air, is provided in order toaccommodate differential expansions or contractions of the housing andthe oil contained therein.

The steps involved in assembling the resistor will now be described.initially, one of the end discs is inserted in the cyiinder and itsmetal ring is soldered in place, making a seal around the periphery. Theresistor unit is connected to the printed circuit board. The leads arebent after being bonded to the respective conductor portions of theprinted circuit board. Preferably, before inserting the assemblyincluding resistor, printed circuit board, and the ultimate resistorterminal leads into the cylinder, this cylinder is lined with a thinlayer of Tefion insulatin material to insure a ainst an accidentalelectrical contact between either of the units and the inner cylindricalwall of the housing.

The lead wire is inserted through the eyelet of disc as thesubassernbly, including the printed circuit board and the resistor ismoved into the interior of the cylinder. When the subassembly isapproximately centrally located within the cylinder, at solderedjunction is formed between the central eyelet of disc and the lead,completing hermetic sealing of the left end of the resistor unit.

Next, the opposite end disc is slipped over a terminal lead wire andmoved into position in the end of the cylinder opposite the first enddisc. The peripheral metal ring of the end disc is soldered to thecylinder around its entire periphery by dipping this end of the cylinder(along with the projecting terminal lead) into a hot solder bath. Duringthis operation, the solder is prevented from bonding the wire to theeyelet of the right hand disc by the heated air escaping through saideyelet from the interior of the cylinder.

The structure is then placed within an evacuation chamber in which is areservbir of the silicone oil. In that chamber, the air contained withinthe structure is substantially completely exhausted, and by virtue ofimmersion of the unit in the silicone, restoration of atmosphericpressure causes the silicone oil to be drawn into the housingsubstantially filling it. Before making the final solder junctionbetween the terminal lead wire and the right-hand end disc, thetemperature of the unit is elevated to approximately C., at whichtemperature the silicone is expanded to a greater than normal volume.Thereafter, cooling of the unit to room temperature results in theingress of a very small amount of air, to provide the desired pocket forexpansion and contraction. The product is completed by forming thesolder junction between the terminal lead wire and the eyelet of the enddisc.

For some ranges of resistance, and for increased heat dissipationcapacity, it is desirable in some instances to use several resistorelements within the housing defined by cylinder and the end discs.

In an alternative construction of a resistor in accordance with theaforementioned Smith application, a single-cavity of a molded plasticcontainer is arranged to accommodate a single film-on-substrate waferresistor, the cavity again being substantially larger in all itsdimensions than the dimensions of the resistance wafer element. Thehousing for the resistor comprises a relatively narrow metal cup openonly at its lower end, and an end closure unit comprising a glass bodyhaving two eyelets bonded thereto by glass-to-metal seals, along with aperipheral metal band also joined to the glass in a glass-tometal seal.

In this form of a resistor, the terminal lead wires are both extendeddownward from the bottom of the resistor unit parallel to each other.

The single-cavity body may omit any printed wiring pattern and relyinstead upon mere passages or bottom holes through which the resistorterminal leads are to extend.

The assembly of the latter arrangement is made up by first bonding theends of the relatively short flexible leads from the film-on-substrateresistor element to the upper ends of the terminal lead wires. Theseterminal lead wires are then passed downward through the cavity of themolded body, and ex tended through the holes in the bottom of said body.The terminal lead wires are then passed through the metal eyelets whichare sealed to the glass of the end closure unit.

A very small layer of ,Teflon, or other suitable insulating material, isinserted within the metal can and made to lie against the closed endthereof. The molded body having the resistor wafer element enclosedtherein is moved up into the interior of the can and the end closureunit is next brought into a position on the lower end of the can, whereit is ready to be soldered in place.

The unit is then soldered by dip soldering, to form a continuous andcomplete bond between the lower end of the can and the metal ring bondedto the periphery of the lower end closure unit and forming a partthereof. In this dip-soldering process, one of the eyelets in the lowerend of the closure unit will be solder-bonded to the terminal lead wirepassing therethrough, but the other will be kept open by the emergingair due to the rising temperature within the can. Again, the process ofsubstantial evacuation is followed by filling with the silicone fillerat the elevated temperature, after which the silicone contracts leavinga small void sufficient to allow for the differential expansion andcontraction of the case and the silicone filling. The final step, inthis embodiment, as in the previous embodiment, is the soldering of theopen eyelet to the terminal wire lead passing therethrough.

The resultant resistor in any of these embodiments is better capable ofstanding shock and vibration than any of the loosely wound wireprecision resistors, and is at least as good as the spool-woundprecision resistors. In contrast to both such types of wire resistors,it has a minimum of reactive effect, its inductance being typically aslow as or lower than one-tenth microhenry, and its distributedcapacitance being typically as low as or lower than one-halfmicro-microfarad. By virtue of such extremely low reactance factors, theresistor in accordance with the aforementioned Smith application remainsat a substantially unity power factor at frequencies far greater than atthe frequencies up to which the resistors constructed of wire may beused, with or without attempts at inductance cancellation arrangements.

Resistors 52-52 are so interconnected to contact arms 48-48 that fourresistors provide the ten settings desired for each switch. For example,four resistors having, respectively, 1 ohm resistance (R 2 ohmsresistance (R 3 ohms resistance (R and 3 ohms resistance (R can becombined in the following combinations to provide from to 9 ohms re- .s1 iqt. .w tcl 1.

Resistor Desired resistance: combinations 4 1 & R

5 & R

6 R & R & R

7 R & R & R

8 R & R & R

9 R &R &R &R

For some applications, apparatus 10 may be modified by connectinginterconnections 32 and 33 directly to each other thereby eliminatingresistor 34. This modification converts the apparatus for solely digitalreadouts of total resistance for resistance settings of 1 ohm variationfrom 0 to 99,999 ohms.

FIG. 5 shows the deviation (AR/R in percent) from various resistancevalues (set on ohmic standard apparatus modified solely for digitalreadouts) at various frequencies. As shown in FIG. 5, at 500 kHz, a 90Kohm resistance setting will deviate only about 5 percent from the setvalue. In conventional resistance decades boxes, a deviation of 5percent will occur at approximately 50 kHz. for the same resistancesetting. Thus, the apparatus of the present invention extends the rangeof useful frequency by as much as 10 times.

Moreover, the apparatus of the present invention eliminates or reducesto insignificance the following problems which are typically found inresistance decade boxes:

a. Resistance shift The shelf stability of the apparatus of the presentinvention can be maintained as low as 5 pp /y b. Resistance Setting Aninfinite resolution in resistance settings is obtained with theapparatus of the present invention which incorporates a slide wire typerheostat. Normally, resistance decade boxes have steps of resistance andwith step type apparatus it is not possible to obtain a continuoussetting.

0. Accuracy The accuracy of the apparatus of the present invention hasbeen shown to be within $0.005 percent at the input terminals. Normally,the accuracy of resistance decade boxes is specified as the accuracy ofindividual resistors across the resistor terminals rather than the inputterminals.

(1. Temperature Coefficient (TC) Tracking The TC tracking for theapparatus of the present invention is 2 ppm/C. over the range of 0 C. to+60 C. for all resistance values above ohms. In conventional resistancedecade boxes each resistor has a different temperature characteristic.

e. Switching Each switch of the apparatus of the present F insiiientionuses only four resistors to provide ten values.

decade boxes, the capacitance at the input terminals of the apparatus ofthe present invention remains essentially constant at 5 pf for allsettings.

g. Size The size of the apparatus of the present invention is as much as2/3 smaller than conventional resistance decade boxes.

As a result of the unusually flat frequency response and othercharacteristics mentioned above, the ohmic standard apparatus of thepresent invention can be used in many applications previously consideredimpractical. Ohmic standard apparatus of this invention is used for:secondary stands; adjustable, direct-reading resistance substitution;components of bridges, voltage dividers, attenuators and multipliers;adjustable feedback resistors for use with operational amplifiers; andladder or network elements.

It will be understood that various modifications can be made withoutdeparting from the invention.

l. Ohmic standard apparatus comprising five switches and four resistorsassociated with each switch by adjustable connecting means; theresistors of the first switch consisting of one l-ohm resistor, one2-ohm resistor and two 3-ohm resistors; the resistors of the secondswitch consisting of one 10- ohm resistor, one 20-ohm resistor and two30-ohm resistors; the resistors of the third switch consisting of onel00-ohm resistor, one ZOO-ohm resistor and two 300-ohm resistors; theresistors of the fourth switch consisting of one l000-ohm resistor, oneZOOO-ohm resistor and two 3000-ohm resistors; and the resistors of thefifth switch consisting of one 10,000-ohm resistor, one 20,000-ohmresistor and two 30,000-ohm resistors.

2. The apparatus of claim 1 wherein the switches are rocker actuatedswitches.

3. The apparatus of claim 1 wherein the resistors comprise resistivemetal film deposited on a rigid dielectric substrate.

4. The apparatus of claim 3 wherein the resistors are mounted inside ahousing substantially impermeable to vapor transmission.

5. The apparatus of claim 4 in which the resistors are mounted inside ahermetically sealed oil-containing housing.

6. The apparatus of claim 1 which also includes a vernier dial markedwith divisions of resistance value and connected to a slide-wireresistor, said slide-wire resistor being electrically connected toswitches.

unt Capacitance Unlike conventional resistance 3 3? I UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 51] Dated M h 23,1971 Inventor-(s) John P. Smith, Jr.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

The address of the inventor should be corrected by deleting "Elton" aninserting Exton Column 4, line 18, change "10" to of Signed and sealedthis 19th day of October 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GO'I'TSCHALK Attesting Officer ActingCommissioner of Pa

1. Ohmic standard apparatus comprising five switches and four resistorsassociated with each switch by adjustable connecting means; theresistors of the first switch consisting of one 1-ohm resistor, one2-ohm resistor and two 3-ohm resistors; the resistors of the secondswitch consisting of one 10-ohm resistor, one 20-ohm resistor and two30-ohm resistors; the resistors of the third switch consisting of one100-ohm resistor, one 200-ohm resistor and two 300-ohm resistors; theresistors of the fourth switch consisting of one 1000-ohm resistor, one2000-ohm resistor and two 3000-ohm resistors; and the resistors of thefifth switch consisting of one 10,000-ohm resistor, one 20,000-ohmresistor and two 30,000-ohm resistors.
 2. The apparatus of claim 1wherein the switches are Rocker actuated switches.
 3. The apparatus ofclaim 1 wherein the resistors comprise resistive metal film deposited ona rigid dielectric substrate.
 4. The apparatus of claim 3 wherein theresistors are mounted inside a housing substantially impermeable tovapor transmission.
 5. The apparatus of claim 4 in which the resistorsare mounted inside a hermetically sealed oil-containing housing.
 6. Theapparatus of claim 1 which also includes a vernier dial marked withdivisions of resistance value and connected to a slide-wire resistor,said slide-wire resistor being electrically connected to switches.